The invention refers to a control system for a hydraulic power lift driven by a variable displacement pump.
There is already known a control system for a hydraulic power lift driven by a variable displacement pump which has a blocking valve that can be opened at least for lowering of the power lift. A load-indicating line controls the load pressure from the pump. A control device switches the one-way valve open and closed and regulates the volumetric flow from a pump line into a load line leading to the power lift (or from the power lift into a tank line leading to a reservoir).
In such control system, a servo valve is provided which: (i) actuates a blocking valve or blocking element using one guiding edge, (ii) actuates a hydraulic component that switches the variable displacement pump (which feeds a hydraulic system) into different operational modes using another guiding edge, (iii) regulates a pump feed flow between a pump line and a service line that leads to the power lift and in which the blocking element rests using another guiding edge with a variable opening surface, and (iv) regulates a reservoir flow between the service line and a tank line using yet another guiding edge with a variable opening surface.
This conventional control system was necessary for a hydraulic system fed by a fixed-displacement pump since in the neutral of the servo valve in such hydraulic systems the fixed-displacement pump had to be switched off and the blocking element rendered closed to avoid high power losses. This control system is also used for hydraulic systems that are driven by variable displacement pumps to reduce power losses.
For the switching and volumetric flow regulating functions of the servo valve, considerable disadvantages result from the obligatory linkage of switching functions and volumetric flow regulating functions insofar as the dynamics and the precision of the control system are limited. This results in a reduced response behavior quality of the control system since when the service line leading to the consuming device or to the power lift is connected, only a gradual pressure build-up in the service line can take place.
To resolve the task of developing the control system in such a way that with simple construction and economical fabrication the response behavior of the power lift is improved, a control system was proposed in the afore-mentioned U.S. Pat. No. 4,960,035. That control system is for a hydraulic lift driven by a variable displacement pump. The system comprises a blocking valve which may be opened to permit a flow of hydraulic fluid to and from the lift. An actuator unit is provided for opening and closing the blocking valve. A servo valve is provided for regulating the volumetric flow to and from the hydraulic lift. The servo valve has three valve positions such that in an initial position, a service line leading to the hydraulic lift is connected to a reservoir. A middle position of the servo valve constitutes a blocking position. The next position of the servo valve connects the service line to the pump. A load sensing or registration line senses fluid pressure coming from the servo valve in order to control the displacement of the pump. The blocking valve and servo valve are controlled by control pressure from a common control line.
The use of the variable displacement pump in conjunction with such a control device allows the switching functions of the control system to be separated from the volumetric flow regulating function. The control system therefore enables different operational demands required of the power lift to be dealt with greater versatility. If precision and reaction speed are required, the variable displacement pump is operated in a load-sensing mode and the blocking element remains open. In operational phases with lesser supply demands, e.g., for driving a vehicle equipped with a hydraulic power lift and under load, the variable displacement pump can be switched in the standby mode by connecting the load-indicating line to a reservoir. By doing so, the blocking valve is automatically closed and the load is thereby secured. As the variable displacement pump is always switched to load pressure before the blocking valve opens, a pre-control of the blocking element can be dispensed with even in the case of high load pressure. This means that the blocking valve can be built much less expensively and furthermore, various hydraulic control lines and elements of the conventional control system are no longer necessary. The combination of the load-sensing mode with the actuation of the blocking valve not equipped with pre-control results in a safety function for controlled and regulated hydraulic systems with a mass load. Since the control pressure level is low compared to the load pressure, the lowering of a large load can only take place from the neutral position of the servo valve. The possibility of sudden dropping of a large load at start-up is also thus precluded when the servo valve is or was inadvertently placed in its lowered position for whatever reason. It is thus possible to carry out (i) the volumetric flow regulation by the servo valve, and (ii) the switching function by which the blocking valve is opened, using a single control pressure, with one part of the control range preferably reserved for the switching function and with the control surface of the control element of the blocking valve being larger than that of the control element of the servo valve.
In the control system disclosed in the aforementioned U.S. Pat. No. 4,960,035, only by optimally improved coordination of the control characteristics of the servo valve on the one hand, and of the control unit 30 with the blocking valve on the other hand, does it become possible to inhibit the occurrence of reactions in the load line downstream from the blocking valve when switching off the control pressure in the neutral position C of the servo valve. The present invention therefore concerns an improvement so that when switching the control pressure closed and open from the neutral position of the servo valve, reactions in the load line downstream from the blocking valve are prevented. At the same time, it should be reliably ensured that the switching measures foreseen here have no influence on the function of the blocking valve when the servo valve passes through the above-mentioned neutral position.